Search devices of this kind have been, for example, presented in the following patent publications: U.S. Pat. No. 3,800,273, U.S. Pat. No. 3,784,805 and U.S. Pat. No. 3,739,324. The first publication discloses an acoustic search device which is to be worn by a swimmer diver. The device comprises a separate viewing mask assembly and a control unit which contains the ultrasonic transducer assembly. A fiber optic image cable is used for transferring the display image from the ultrasonic transducer to the optical viewing device. The most efficient mode of operation is to use the control unit like a hand held electric light. The patent publication U.S. Pat. No. 3,784,805 discloses an acoustic search device which similarly comprises a face mask with an optical viewing device and a separate ultrasonic transducer unit which contains electroacoutical transducer elements suitable for both transmitting and receiving acoustic signals. The display image from the ultrasonic transducer is transferred to the optical viewing device inside the face mask via a fiber optic cable. The patent publication U.S. Pat. No. 3,739,324 discloses an acoustic system for viewing underwater objects and a way for obtaining a depth perception by means of a rotating transparent disk in the optical viewing device. The display image is viewed through the disk which is positioned in front of one eye. The rotary motion of the disk is synchronized with the operation of the ultrasonic transmitter.
The drawback of the above acoustic search devices is the fact that their use is almost impossible in circumstances where, on one hand, the image seen must correspond to reality and, on the other hand, the device must be of such simple and compact construction that it does not make the movements and operations of the user difficult.